The YAML module allows you to use one of the two YAML engines that ship with ruby. By default Psych is used but the old and unmaintained Syck may be chosen.
See Psych or Syck for usage and documentation.
To set the YAML engine to syck:
YAML::ENGINE.yamler = 'syck'
To set the YAML engine back to psych:
YAML::ENGINE.yamler = 'psych'